1. Field of the Invention
The present invention relates to detection of substrates, such as semiconductor wafers, within a cassette, and conveyance of the substrates from and into the cassette.
2. Description of the Related Art
In a substrate processing apparatus, such as a semiconductor wafer processing apparatus, each wafer held in a cassette is taken out from the cassette with a conveyance arm. According to conventional technique, the positions of the substrates within the cassette are first detected, and then the conveyance arm is horizontally inserted between the substrates.
A conventional apparatus for detecting substrates within a cassette checks whether a light receiving element of a light sensor is in a light-receiving state or in a non-light receiving state at each predetermined position in a cassette, thereby determining whether or not a substrate exists at each predetermined position.
The cassettes have grooves for receiving substrates. The grooves typically give a rise to their rear portions. Further, the cassettes themselves may be distorted. Thus, the substrates within such a cassette are slightly inclined forward in many cases.
In some cases, a clearance between two substrates within a cassette becomes less than a thickness of a conveyance arm. If the conveyance arm were horizontally inserted between the two substrates, the arm would interfere with the two substrates.
In order to allow the conveyance arm to be inserted into a cassette without interfering with any substrate in the cassette, it is accordingly desirable to detect a vertical positional range of each substrate, in addition to the information showing whether or not a substrates exists at each predetermined position. The prior art detection apparatus, however, can not detect the vertical positional range of each substrate within the cassette.